about:configAnd scroll down to browser.tabXXXXXX From there its a question off trial and error.

Keep in mind that Firefox is quite unstable about most changes.Weird as it sounds, one might have to reset old config settings and try the same again. And then it works. (Is that called a bug perhaps... though this is a well known feature in the community)

Edit: If you have not yet tried it.Full-screen-mode is really an enhancement of Firefox anf hides title-bar and tabs in a second stage.Just a tip for its a goodie..... ;-) [F11]